1. Technical Field
The present invention relates to antennae and receivers and more particularly to duplexers used in radar systems for simultaneously transmitting and receiving signals to and from an antenna. Even more particularly, the invention relates to the use of high dynamic range low noise amplifiers in combination with directional couplers of a cancellation circuit to cancel leakage signals in order to provide greater sensitivity of operation of the radar system.
2. Background Information
In radar apparatus, duplexers isolate the receiver from the transmitter while permitting them to share a common antenna. A duplexer is designed for operation in the frequency band used by the receiver and transmitter, and must be capable of handling the output power of the transmitter. The duplexer must provide adequate rejection of transmitter noise occurring at the receive frequency, and must be designed to operate at, or less than, the frequency separation between the transmitter and receiver, and must provide sufficient isolation to prevent receiver desensitization. Heretofore, this technology has been limited by an inability of duplexers in transmit/receive modules to allow simultaneous transmit and receive. Transmit leakage through isolation and antenna port reflection paths as well as mutual coupling from adjacent antenna elements in a phased array, overloads the first amplifier stage in the receive path.
This problem has been recognized in the past and has been attempted to be corrected by various types of cancellation circuits such as described in U.S. Pat. No. 3,021,521. The apparatus of this patent solves some of these problems. However, it completely cancels the receive signals near the transmit frequency so that simultaneous operation is not possible. It also may have possessed additional shortcomings which are discussed in detail in U.S. Pat. No. 4,968,967 which discloses a radar system which solves many of the problems of the system and apparatus of U.S. Pat. No. 3,021,521, and in addition allows for the simultaneous transmit and receive operation as that of the present invention.
Although the system of U.S. Pat. No. 4,968,967 is believed to solve many problems and allows simultaneous transmit and receive operations, it incorporates no amplifier in the path between the antenna and receiver. Any losses between the antenna and the receiver (or first low noise amplification device) directly degrade sensitivity by adding thermal noise proportional to the losses directly into the receiver. In addition to the path losses (such as transmission lines or losses in the coupler), the coupler directly introduces noise into the receiver from the cancellation path and the terminated port of the coupler.
Thus, there is a need, therefore, to provide a radar system, and in particular a duplexer therefore, which provides simultaneous transmit and receive capability with greater sensitivity of operation then existing radar systems which provide for simultaneous transmit and receive feature.